Polystyrene is useful in a number of applications including packaging and, in particular, food packaging. Further, polystyrene is also useful in a number of foam applications such as insulated foam drinking cups or containers for individual servings of prepared food (e.g. "clam shells"). Polystyrene has a glass transition of about that of boiling water (100.degree. C.). In some restaurants, to try to keep hot drinks hot, such as coffee, the serving temperature approaches that of the glass transition temperature of polystyrene. There is a need for a method for increasing the glass transition temperature of polystyrene.
The glass transition temperature of polystyrene may be increased by physical blending with a polymer having a higher glass transition temperature. However, it is expensive to pass material through an extruder to try to obtain a "true solution" of such a high glass transition temperature polymer in polystyrene (i.e. a material which exhibits only one glass transition temperature).
It is also possible to copolymerize styrene with a co-monomer which will result in a polymer having an increase in the glass transition temperature over that of polystyrene. One such polymer is maleic anhydride. However, Maleic anhydride has a high melting temperature and it is difficult to get it into and keep it in a melt of styrene/polystyrene. If it does precipitate before it forms a copolymer, it can necessitate the cleaning of a reactor resulting in losses due to down-time.
There are a number of patents assigned to General Electric, Company relating to a process for producing foamable beads to make a foam having a good resistance to heat. However, these references disclose a suspension process. The monomers are suspended in water at some stage during polymerization. This results in the incorporation of surfactants into the resulting polymer and a requirement to treat the water (continuous phase) in the emulsion process prior to discharge. Representative of such art are U.S. Pat. Nos. 4,782,098; 4,874,796; 4,920,153; and 5,008,298.
A similar approach has been taken by BASF as illustrated by U.S. Pat. Nos. 5,093,374 and 5,093,375.
World Patent Index abstract of JP 05310864 assigned to Mitsubishi Kasei Polytec Co. also takes the same approach.
All of the above art taken together strongly suggests that the trend in the industry to make the type of polymer in accordance with the object of the present invention is towards the use of a suspension process and not a bulk or solution polymerization process.
U.S. Pat. No. 3,664,977 teaches blending polyphenylene ether into the bulk polymerization of high impact polystyrene (HIPS) at a point after inversion of the solution of styrene in rubber (e.g. when the amount of polystyrene in the solution is about equal to the amount of rubber in the polystyrene) up to about 40% conversion. The reference does not teach blending polyphenylene oxide into the styrene monomer at the commencement of the polymerization. Again the reference teaches away from the process of the present invention.
ATOCHEM (formerly Pennwalt Corporation) is a supplier of a number of peroxide initiators to the polymer industry. Two references which describe the use of organic free radical initiators are the article "New initiators for PS offer big efficiencies" by V. Kamath, Modern Plastics, September 1981, pp. 106, 108, 110 and U.S. Pat. No. 4,125,695 both of which disclose the use of organic peroxide initiators to polymerize polystyrene. However, the references do not suggest the initiators could be used to form a true solution of polyphenylene ether in polystyrene.
The present invention seeks to provide a simple direct efficient process to produce a solution of polyphenylene ether in polystyrene.